Recent applications of glass cloth used in a printed circuit board have widened to the development of package applications. Many packages have a square shaped XY plane and a substrate called an interposer used in this package is required to have no anisotropy in the XY directions.
For these packages, generally, a polyimide film based substrate or a substrate impregnated with a matrix resin and glass cloth as a reinforcing material is used. In view of the requirements for such an application, such as high rigidity, high dimensional stability, low thermal expansion, and the like, the use of a film substrate using glass cloth has been increasing. However, anisotropy in the XY directions of the film substrate, expressed by structural differences between the warp yarn direction and the weft yarn direction derived from the glass cloth, has now become a problem. At the same time, because of a requirement for a thinner package, a thinner interposer itself is needed and thus the thickness of the glass cloth used is required to be as thin as 50 μm to 20 μm.
For such a thin glass cloth, those with a reduced clearance rate by narrowing yarn distance by a fiber-opening treatment have been proposed in JP-A-5-286055, JP-A-8-18179, JP-A-11-114956 and JP-A-2002-38367.
However, the invention described in JP-A-5-286055 aims at the suppression of the dimensional change rate in a multilayer plate molding and in the Examples thereof, a multilayered plate composed of a 6 layered prepreg is described, but there is no description of a one-layer film substrate.
Also the invention described in JP-A-8-18179 aims at the improvement of the heat resistance in soldering and there is no description on the effects of anisotropy in the XY directions.
The invention described in JP-A-11-114956 also aims at the prevention of pore generation in a resin during prepreg preparation and there is no description on the effects of anisotropy in the XY directions.
Furthermore, in the invention described in JP-A-2002-38367, there is a description that suggests a reduction of anisotropy in the XY directions by increasing the opening rate in the XY directions. However, sufficient reduction of anisotropy in the XY directions could not be attained, as shown by the difference between the opening rate of the warp yarn and the opening rate of the weft yarn in the Examples described in Table 1 or 4. As is also clear from the description on the dimensional change rate between the width and the length directions in the Examples described in Table 1 or Table 2, there is no description of a one-layered film substrate, although a multi-layered plate composed of a 4 layered prepreg is described.